Structures and Dissociation Products of Ce/Peptide Complexes: Competition between Coordination and Charge Delocalization

Abstract

Structures of [Ce­(GGG)]3+ and [Ce­(GGG – H)]2+ have been investigated by DFT calculations. The two lowest-energy structures of the triply charged metal complex have the peptide in either the iminol or conventional zwitterionic form, and these ions have almost identical energies. In the doubly charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface, but the latter is favored. In both iminol structures, the metal ion coordinates to the iminol oxygen rather than to the nitrogen, unlike in previously reported iminol-containing complexes. Triply charged [Ce­(peptide)]3+ complexes are fragile and not easily isolated in a mass spectrometer, whereas the doubly charged [Ce­(peptide – H)]2+ complexes are more robust. Here, we studied the fragmentations of 37 [Ce­(peptide – H)]2+ and 30 [Ce­(peptide)­(peptide – H)]2+ complexes and the results are systematically summarized. Losses of CO and/or H2O are the most commonly observed fragmentation channels for [Ce­(peptide – H)]2+ complexes and these dissociation pathways are modeled by DFT calculations. For [Ce­(peptide)­(peptide – H)]2+ complexes the neutral peptide plays the role of a solvent molecule but, unlike in the dissociations of [Ce­(CH3CN)­(peptide – H)]2+ complexes, the loss of the solvent molecule is not observed. Instead, fragmentation occurs by cleavage of the second amide bond of the solvating peptide molecule.